This invention relates to glass-to-metal seals and, more particularly, to glass compositions that form hermetic seals with titanium and titanium alloys. These seals can be used in components requiring the use of titanium housings or feedthroughs, e.g. in seals for battery headers. Titanium and titanium alloys have a number of desirable properties, including a high strength-to-weight ratio, high resistance to chemical attack, and moderate high temperature stability. The availability of pure titanium and titanium alloys in both quantity and shape or form has increased over recent years, making its use more attractive and feasible.
In order to form an acceptable hermetic seal with titanium, the glass must meet three main criteria. First, the glass must have a coefficient of thermal expansion (CTE) that matches titanium over the range from room temperature to the glass transition temperature. The second criterion is that the glass must be chemically compatible with the titanium. Titanium is a very reactive metal at high temperature, and reactions between titanium and molten oxide glasses are generally thermodynamically favorable. If the reaction generates gaseous species at the interface, hermeticity and mechanical integrity are difficult to retain. The final requirement is that the glass must be able to seal to titanium at temperatures below 882.degree. C. At this temperature, titanium exhibits an allotropic phase transformation that is not fully reversible upon cooling. This results in changes in thermal properties and degradation of low temperature mechanical properties.
Up to now, titanium and titanium alloys could not be used in components requiring glass-to-metal seals. Conventional alkali silicate sealing glasses with the required matching coefficients of thermal expansion can react with titanium at the seal temperature to form bubbles at the glass-metal interfaces, thus degrading the seal performance. Alkali volatilization has been reported in seals between titanium and alkali silicate glasses (I. J. McColm, C. Dimby Low, Journal of Materials Science, Vol. 9, pp. 1320-1324 (1974)). In addition, Ti silicide formation has been observed (A. Passerone, G. Valbusa, E. Biagini, Journal of Materials Science, Vol. 12, pp. 2465-2474 (1977)) and characterized to have poor adherence with the glass. Finally, these glasses require high sealing temperatures that cause the titanium phase transition that can affect detrimentally some of its advantageous mechanical properties.
Although the possibility of creating glasses for sealing to titanium has been previously discussed in the relevant literature, the required technology has not been available in the past. Rawson and Denton described the properties of titanium sealing glasses in 1954 (H. Rawson, E. P. Denton, British Journal of Applied Physics, Vol. 5, pp. 352-353 (1954)). The CTE's of their glasses matched titanium, but high sealing temperatures (900.degree.-1000.degree. C.) were required. Also, those glasses contain alkali oxides and silica, species that are unstable with respect to titanium at high temperatures. Therefore, very short sealing times were required to prevent appreciable reaction of the glass with titanium, and to minimize the extent of the titanium phase transformation.
Kimble glass literature (Kimble Glass/Owens Illinois Technical Data Book) recommends several glasses for sealing to titanium, based only on thermal expansion capability however. KG-12 is an alkali-lead-silicate glass requiring temperatures of 900.degree.-1000.degree. C. for sealing. TM-9 is a commercial alkali-barium-silicate glass that also requires temperatures above 950.degree. C. for sealing. EG-6 is similar to KG-12. The use of any of these glasses will promote the titanium phase transformation as well as the deleterious reduction of alkali oxides and the formation of deleterious interfacial titanium silicides. Thus, there is no previously established technology that describes hermetic sealing to titanium that meets the above-described criteria.
There is an existing need for the development of such technology to allow the use of titanium and titanium alloys in explosive and pyrotechnic components requiring hermetic seals, in components requiring high strength pins or casings, and in components susceptible to corrosion.
Also, there is an existing need for the development of titanium sealing glasses for wide commercial applications in biomedical implant devices, such as pacemaker batteries, where the outstanding corrosion resistance of the titanium would be of prime importance.